Particles render in a different size
I have found the issue, it seems to be a bug, I reduceed the particle ammount to 2000 from 20000 and it all worked out. As you intended.

I tried to figure out how many the maximum ammount of particles there can be, by gradually increasing the particle ammount, 19999 is fine, then at 20000 it gliches out and is normal at 20001 again, I have absolutely no clue how this is possible and why this can happen but my guess is it being a bug.